In newer digital optical communication systems operating at higher speeds of bit rates at 10 Gbits/sec or more, monitoring the signal quality deterioration becomes more problematic with increasing bit rates. Conventional methods involving the electronic measurement and subsequent processing of the optical signal eye diagram will be limited by the inherent difficulties in performing electronic processing at very high bit rates, rendering such methods to be prohibitively expensive for bit rates in excess of about 10 Gbits/sec.
Examples of typical eye diagram measuring techniques, which involve regenerating an optical signal to monitor signal quality, are disclosed by Harman in U.S. Pat. No. 4,097,697 and by Tremblay et al in U.S. Pat. No. 4,823,360.
It is also known from H. Takara et al, “Eye-diagram measurement of 100 Gbit/s optical signals using optical sampling”: 22nd European Conference on Optical Communication—ECOC 1996 Oslo, vol. 4 pages 7-10, that eye diagrams of very high-speed optical signals can be obtained by optical sampling using an organic non-linear crystal. The ability to use optical sampling before conversion to electronic signal facilitates the use of less sophisticated electronic processing: Reliance upon the organic non-linear crystal, however, has inherent disadvantages such as the difficulty of integrating this optical component in a sensor system. A further major disadvantage of this method is that sampling pulses are required to be generated at excessively high power, i.e. in excess of 200 watts.
It is further known from Idler et al, “10 Gb/s Wavelength Conversion with Integrated Multiquantum-Well-Based 3-Port Mach-Zehnder Interferometer”: IEEE Photonics Technology Letters, Vol. 8, No. 9, September 1996, pages 1163-1165, that inversion of a single optical signal in addition to wavelength conversion can be provided by means of a Mach-Zehnder interferometer in which semiconductor optical amplifiers are utilised to set an interference condition between optical components of an input signal transmitted through first and second arms of the interferometer. A continuous wave optical signal propagated equally through the first and second arms is recombined to form an interference signal which is modulated according to the interference condition and a pulsed optical signal is counter-propagated through only one of the arms so as to modulate the phase of one of the component signals by cross-phase modulation due to the non-linear characteristics of the semiconductor optical amplifier in that arm.
More recently, Roberts disclosed in U.S. Pat. No. 5,880,837 a technique for monitoring signal deterioration of an optical by obtaining eye measurement data, using an interferometer for optical sampling of the optical signal. Sampling optical pulses are propagated equally through the two interferometer arms, where each arm includes semiconductor optical amplifiers. The monitored optical signal is counter-propagated through one arm of the interferometer, thereby setting an interference condition of the interferometer by cross modulation in one of the semiconductor optical amplifiers. The sampled interference signal is detected and converted to electrical signals to be processed for obtaining eye measurement data. Again the use of semiconductor devices in this disclosure may present practical limitations with optical signals at very high bit rates.
In view of the limitations in the prior art reviewed above, there still remains a clear need for simplified means for characterizing signal quality deterioration of high-speed digital optical signals having bit rates of 10 Gbits/sec or more without having to rely on the use of semiconductor devices.